Cycles - CUDA CPU Extremely Slow When Combined With GPU
This is normal. It is common for GPUs to be a lot faster at rendering than CPUs and in this case GTX 1070 is definitely a few times faster than i7(?) 6700K so for renders that take seconds, this is to be expected. You would probably benefit from combining GPU and CPU on longer renders(think minutes and hours, not seconds), but 20 seconds is short enough time that the render would get stuck on slower CPU tiles at the end when the GPU has nothing else to do. Managing the render process on a few devices probably also takes extra time. You could try using really small tiles and see if that helps, if not, just use the GPU alone.